1. Field of the Invention
The present invention relates to semiconductor memory devices. In particular, one embodiment of the present invention relates to a semiconductor memory device including a memory cell including a transistor whose channel region is formed of an oxide semiconductor.
2. Description of the Related Art
In recent years, a metal oxide having semiconductor characteristics, which is called an oxide semiconductor exhibiting a high mobility and uniform element characteristics, has attracted attention as a material of a transistor. Metal oxides are used for a variety of applications. For example, indium oxide is used as a material of a pixel electrode in a liquid crystal display device. Examples of such a metal oxide having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide; transistors which include such a metal oxide having semiconductor characteristics in their channel regions are known (Patent Documents 1 and 2).